Field of the Invention
The invention pertains to an air-conditioning installation for aircraft, especially passenger aircraft.
The air supply for the passengers of an aircraft flying at high altitude is provided by an air-conditioning installation that uses external air at low pressure, often at about 280 millibars, to deliver air into the cabin at a pressure of about 800 mb and a temperature of about 0.degree. C. (for a cabin generally has to be cooled down because of the heat released by the passengers themselves). For this purpose, the installation has an inlet compressor receiving external air and delivering power-compressed air (hence heated air) either directly into the cabin (if the cabin has to be heated) or into a cooling unit. A cooling unit of this type has a second compressor followed by a heat exchanger subjected to an external air current, for example at a temperature of about 10.degree. C., in order to cool the air. The compressed air that leaves the exchanger is then power-expanded (so that it can be cooled) by means of an expansion turbine mounted on the same shaft as the compressor in order to bring it to the desired pressure and temperature when it leaves the unit.
To adjust the temperature if it is too low, an adjustment is made in the size of the opening of a by-pass valve between the exchanger and the aircraft cabin.
The energy expended to deliver compressed air to the inlet of the cooling unit is significant and desirable to reduce this energy expenditure to a minimum.
The invention can be used to reduce this consumption of energy to the minimum.
For this purpose, the invention is based, firstly, on the fact that the air in the aircraft cabin in flight is at a greater pressure than the ambient pressure and, secondly, on the fact that the efficiency of the air-conditioning installation is all the greater as the coldest point of this installation is at a lower temperature.
According to an embodiment of the invention, the air that leaves the cabin is conveyed to an expansion turbine that gives cold air which is mixed with the cooling air of the exchanger to reduce the temperature at the inlet of the turbine of the cooling unit and limit the lowering of the temperature through the turbine wheel and even eliminate it in certain cases. This makes it possible to reduce the power of the inlet compressor and to do away with the use of an expansion turbine after this inlet compressor.
It is true, that, in the prior art, it has already been proposed to use a turbine for the expansion of the air leaving the cabin in order to produce cold air (for example, in the U.S. Pat. Nos. 4,419,926 and 3,369,777). But the installations proposed up to now are less efficient than the installation of the invention where a turbine is used to produce air at a lower temperature than that of the external air.
According to another embodiment of the invention, which also uses a turbine to expand the air leaving the cabin and which produces air at a temperature lower than that of the external air, the installation comprises an additional heat exchanger placed between the outlet of the exchanger cooled by the external air and the inlet for introducing air into the aircraft cabin, the said additional exchanger being cooled by the air given by the expansion turbine.
The difference between the temperature of the external air and the temperature of the air given by the expansion turbine is, for example, 30.degree. C.
The expansion turbine, actuated by the air that leaves the cabin, preferably drives a compressor receiving outside air, the action of the said compressor being added to that of the inlet compressor. Like this inlet compressor, it gives air at the inlet of the cooling unit. Thus, the power of the inlet compressor is even further reduced.